The present invention relates to an evaporator adapted for a counter-current flow of at least one liquid and vapor therein. The present invention also relates to a process for using said evaporator to separate at least two components and the use of said evaporator in the purification and/or concentration of a thermally-sensitive compound and/or in the removal of a solvent.
An evaporator is a device that transforms a liquid material into a vapor form, and evaporators may be used to separate compounds based on their relative boiling points and volatility. Evaporation processes are of utility, for example, in the work-up after a chemical reaction to isolate, concentrate and/or purify the product(s) in a variety of industrial processes. Evaporation is thus widely used to concentrate foods and chemicals as well as to recover solvents. The purified or concentrated substances may be inorganic in nature such as metallic compounds, organic such as fine chemicals or natural products, or organometallic compounds. The removed substances may be water, solvents and/or reaction byproducts or other impurities.
In a typical evaporation system, a liquid containing the desired product is fed into an evaporator and is heated by a heat source. The applied heat converts a volatile component in the liquid into vapor. The vapor is removed from the rest of the liquid in a separator and then condensed by a condenser. The thus concentrated liquid product may be either fed into a second evaporator unit, recirculated or removed. The evaporator may be operated at reduced pressure of typically about 2 to 10 mbar absolute so as to favorably reduce the boiling points of the components, thus allowing temperature-sensitive products to be purified and/or concentrated at lower temperatures.
Various types of evaporators are known, such as the falling film, flash, rising film (long tube vertical), climbing and falling plate, wiped film, and multiple effect evaporators. For example, US 2004/0182692 A1 discloses a falling film evaporator. Typical of such evaporation systems is the use of a series of equipment that provide standard units of operation, for example, an evaporator unit followed by a condenser unit, optionally with an intervening separator unit. These separate units are contained in pressure vessels with their own individual shells and connected together by piping. The use of longer piping having smaller diameters and with more bends generates more pressure drop and thus limits the vacuum level that may be achieved in the evaporator. For example, a typical vapor line between an evaporator and condenser may cause a pressure drop of about 1 mbar, which is then on the same order of magnitude as the operating pressure of the evaporator itself. Such typical pressure drops may undesirably increase the pressure in the evaporation section by about 10% to a factor of about 3. Furthermore the use of such separate units makes the system more costly and complex to design and operate, as well as causing it to have a large room requirement (“footprint”).
More elaborate evaporator systems are improved by using a mist eliminator unit between the evaporator unit and the condenser unit or by using a feed preheater unit upstream of the evaporator unit for heat recovery. The feed preheater unit is in general heated with the liquid that is leaving at the bottom of the evaporation unit. However, in cases where most of the feed is evaporated, the largest fraction is the vapor fraction. Feed preheating by means of the vapor fraction is more challenging as a partial condenser is required. This partial condenser creates even more pressure drop for the vapor that flows in total through the evaporator, demister, feed preheater and condenser. Therefore attempting to improve the evaporator by the addition of these additional units limit the vacuum that may be achieved and thus higher operating temperatures are required which unnecessarily damages thermally-sensitive compounds.
It is noted that wiped film evaporators may be used for treating thermally-sensitive compounds; however, such evaporators still require the use of several shells and they also rely on moving parts, which requires both increased maintenance and investment costs.
In conclusion, it would be desirable to have an evaporator capable of operating at lower pressures and thus also temperatures and that would thus be better suited for purification of thermally-sensitive compounds, especially those having low viscosity (i.e. less than or equal to about 20 millipascal-second), than those of the prior art, and while also offering an integrated system not requiring piping between sub-units and separate pressure vessel shells for each sub-unit thus reducing the cost, complexity and footprint.